This invention relates to an afterburner flameholder and, more particularly, to an afterburner flameholder for inclusion within the high velocity gas stream of an aircraft gas turbine engine as part of an afterburning system to provide additional thrust augmentation.
It is well known in the aircraft gas turbine art to provide thrust augmentation by burning additional fuel in an afterburner located downstream of the engine turbine. The afterburner generally includes means for dispersing a main flow of fuel together with a flameholder to which the flame may attach. The flameholder reduces locally the velocity of the gas stream and establishes a recirculation zone within the afterburner in order to sustain the flame which would otherwise blow out. The flameholder further provides an ignition and low temperature rise zone which, in conjunction with additionally injected fuel in parallel and sequentially to a pilot fuel flow, provides staging to accomplish broad temperature modulation of the afterburner. One well known type of flameholder comprises two concentric flame rings arranged to diverge from each other in a downstream direction. Fuel may be introduced either uniformly upstream of the flameholder or in locally concentrated manner so that fuel droplets impinge upon the outside diverging surfaces of the flameholder and the afterburning flame attaches to the trailing edges of the flame rings.
In order to provide for positive and uniform lightoff of the afterburner during all modes of flight operation, pilot fuel may be introduced and sparked to ignite by means of a point source igniter. The pilot flame, in turn, operates to ignite the main fuel droplets. It is well known to introduce the pilot fuel to the afterburner by means of discrete jets situated around the flameholder. The pilot fuel jets are generally located intermediate the flame rings such that each pilot jet receives gas flow from the turbine exhaust through an inlet to the flameholder.
One afterburner flameholder of this type is described in U.S. Pat. No. 3,765,178 issued to Robert Hughes Hufnagle, et al. on Oct. 16, 1973, and assigned to the same assignee as the present invention. The afterburner flameholder of the Hufnagle, et al. patent includes an inner flame ring and an outer flame ring spaced radially outwardly from and concentric to the inner flame ring to form an annular flow passage for receipt of the turbine exhaust. An inlet screen comprising a plurality of circumferentially spaced apart tubes is provided between the inner and outer flame rings closely adjacent the annular inlet thereto in order to meter the flow between the flame rings and precipitate turbulence in the wake behind the screen and thereby minimize circumferential temperature gradients in the incoming gas. Further, at the outlet to the annular flow passage, circumferentially spaced apart air foil type swirl vanes, radially extending between the inner and outer flame rings are provided in order to produce rapid propagation of flame around the entire flameholder and to provide a surface for holding the flame.
Flameholders of the type described in the Hufnagle et al. patent provide a flowpath for the hot turbine gases which is smooth and without abrupt changes or discontinuities so as to reduce the risk of "flashback" and "preignition".
Flashback can generally be described as an upstream propagation of the afterburner flamefront into the interior of the flameholder in the area between the flame rings, and may occur if the velocity of the flow through the flameholder falls off locally below the minimum velocity required to maintain flame attachment. Preignition is not as well understood as flashback, and relates to a sudden spontaneous ignition of the flow in the area between the flame rings of the flameholder. Preignition differs essentially from flashback in that the upstream preignition flame inside the flameholder exists independently of the downstream flame which remains attached to the flame ring trailing edges. preignition may be a direct result of temperature gradients in the flow through the flameholder. The deleterious effects from flashback and preignition are essentially the same, resulting in a premature localized burning of the flameholder components.
While minimizing the occurrence of flashback and preignition, afterburner flameholders of the type disclosed in the Hufnagle et al. patent have several disadvantages. One such disadvantage is that the airswirl vanes located at the outlet to the annular flow passage have demonstrated poor flameholding characteristics. Accordingly, flameholders of the type disclosed in the Hufnagle et al. patent have been subjected to flameout under various operating conditions. Another disadvantage of flameholders of the type disclosed in the Hufnagle et al. patent is that the outer edges of the swirl vanes which come in contact with the flame have not been able to sustain the high temperatures to which they have been exposed and accordingly have been subject to structural damage due to overheating.
It is therefore an object of this invention to provide an afterburner flameholder wherein the flowpath therethrough is smooth and uniform to thereby reduce the risk of flashback and preignition.
It is also an object of this invention to provide an afterburner flameholder having a relatively high internal air velocity so as to preclude flashback and preignition.
It is a further object of this invention to provide an afterburner flameholder which exhibits highly stable flameholding characteristics.
It is a further object of this invention to provide an afterburner flameholder which utilizes a system of swirl vanes to create circumferential flow to provide uniform fuel air mixture, and to establish shear turbulence behind the flameholder to enhance the burning characteristics of the afterburner.
A further object of this invention is to provide an afterburner flameholder in which the swirl vanes are out of contact with the flame so that they are not susceptible to high temperature structural damage.
A further object of this invention is to provide an annular recirculating V-gutter downstream of the swirl vanes to provide a strong recirculation zone for stable combustion of a portion of the carbureted fuel air mixture.
It is still a further object of this invention to provide an afterburner flameholder which uses a system of radially extending V-gutters to improve its flame spreading characteristics.